Functional and Chemical Characterization of B‐Cell Growth Factor Produced by Normal Cloned T Helper Cells

Media conditioned by clones of normal helper T cells exposed to appropriate antigenpresenting cells contain growth‐promoting activity for B‐cell blasts induced either by lipopolysaccharide or on direct interaction with competent helper cells. This B‐cell growth factor (TH‐BGApet) is recovered on sodium dodecyl sulphate polyacrylamide gel electrophoresis corresponding to mol. wt of 15,000–20,000 displays no mitogenicity for small, non‐induced B lymphocytes and is completely devoid of the ability to activate immunoglobulin secretion in proliferating B cells. These results are ascribed to the activity derived from normal T cells, with the same characteristics as BSF‐pl previously obtained from lymphomas and hybridomas. Since hybridization of these T helper cells results in the constitutive production of BSF‐pl in the absence of macrophages, these experiments demonstrate that BSF‐pl is a normal T‐cell product.